Little is known about the virulence of Legionnaires' Disease bacillus (Legionella pneumophila) and its affects on the host, despite the fact that this organism has now been identified as a human pathogen. There are very few reported studies concerning the immune response to this microorganism, in either man or experimental animals, except for reports of a rise in antibody titers to this bacillus after infection or experimental immunization. Thus, this proposed investigation is concerned with the nature and mechanism of cellular (T lymphocyte and macrophage) immune responses following exposure of guinea pigs to the organism or antigens derived from the organism. Cellular immune responses will be compared to humoral B lymphocyte antibody formation. Immune responses at the level of T lymphocytes and phagocytic monocytes will be studied by a variety of immunobiologic techniques, including blastogenesis, lymphocyte migration inhibition and chemotactic assays as a function of time after infection or immunization. The effects of the bacillus on immune responsiveness per se (i.e., immunoregulatory effects) will also be determined in the animal model system. It has been suggested that predelection for infection with L. pneumophila occurs in immunosuppressed patients and, furthermore, that following infection patients appear to continue to show a level of immunderangement. Thus guinea pigs infected with viable organisms or extracted antigens will be examined for immunodeviations and, if this occurs, the mechanism(s) will be examined by appropriate cell transfers and in vitro culture techniques to determine whether specific suppressor or regulator cells or soluble suppressor factors are involved. These studies are relevant to the peculiar characteristics observed in outbreaks of Legionnaires' Disease infection and should provide important information concerning cellular immune responses as compared to humoral immunity to these bacteria and their products.